In electrophotography, an electrostatic latent image formed on a photoreceptor is generally developed with a toner containing a pigment, etc., and the resulting toner image is transferred to a transfer sheet and fixed thereon by a hot roll, etc. The photoreceptor is subjected to cleaning for formation of a next electrostatic latent image.
Dry developers used in electrophotography, etc. are divided into one-component developers solely composed of a toner comprising a binder resin having dispersed therein a colorant and two-component developers composed of such a toner and a carrier. In order that these developers have process suitability in copying, they are required to be excellent in fluidity, anti-caking properties, fixability, chargeability, cleaning properties, etc. To improve these properties, particularly fluidity and anti-caking properties, inorganic fine particles are frequently added to a toner.
However, these fine particles have considerable influences on charging properties. For example, generally employed silica type fine particles have strong negative polarity, which seriously reduces chargeability of positively chargeable toners, and also cause great variations of chargeability with environmental changes between summer and winter, often resulting in background fog and insufficient density reproduction.
Dispersibility of the inorganic fine particles also has great influences on toner characteristics. Particles of poor dispersibility tend to fail to obtain desired effects of improving fluidity and anti-caking properties or tend to cause adhesion of toner particles to a photoreceptor due to insufficient cleaning, resulting in image defects such as black spots.
In order to overcome these problems, it has been proposed to use surface-treated inorganic fine particles. Examples of surface-treated inorganic fine particles so far proposed for positively chargeable toners include silica treated with a charge control agent capable of controlling charge polarity of toners to positive (see JP-A-55-135854 (the term "JP-A" as used herein means an "unexamined published Japanese patent application")); silica fine particles treated with a quaternary ammonium salt (see JP-A-63-143562); inorganic particles having thereon a silicone rubber having an amino group or a hardened product thereof (see JP-A-63-155149); inorganic fine particles treated with an amino compound after having been rendered hydrophobic (see JP-A-63-155152); and inorganic fine particles treated with polysiloxane containing an ammonium salt as a functional group (see JP-A-1-123252).
Any of the toners using these treated inorganic fine particles nevertheless has its own merits and demerits. Taking, for instance, the silica particles disclosed in JP-A-55-135854 supra, since the charge control agent used is a dye, it colors the silica powder, making application to color toners difficult. None of the others satisfies all the characteristics required.